Lithium ion batteries (LIB) are widely used as the ubiquitous power source for portable electronic devices, such as cell phones and laptop computers. Graphite is the most widely used negative electrode material for the rechargeable lithium ion batteries. However, the energy density of graphite is relative low, i.e. only 372 mAh g−1. To further enhance the energy density of lithium ion batteries, researchers focus on silicon based anode materials due to their high lithium storage capacity. However, the main problem of these materials is pulverization and loss of electronic conductivity of the electrode resulted from huge volumetric change during lithiation/delithiation. To solve such a problem, nanosized and porous structured materials are chosen to buffer the volume expansion related to the alloying with lithium, and thereby overcome the mechanical failure of the electrode. Among the Si based anode materials investigated and explored, silicon monoxide (SiO) appears particularly promising because of its long cycle life and low cost. The better cycling performance of SiO is originated from the formation of lithium oxide and lithium silicate, acting as buffer layers to minimize the volume changes during the charging and discharging.
The volume expansion of SiO is about 200% upon electrochemical lithiation of about 2600 mAh g−1, comparing with silicon, which is 400% for lithiation of 4000 mAh g−1. Since the volume explain of SiO during lithiation is suppressed by the formation of lithium oxide (Li2O) and lithium silicate (Li4SiO4), the high cycling performance of SiO could be easier to achieve than that of silicon. And for practical application, anode capacity of 1000-1500 mAh g−1 would be enough for current cathode materials. Therefore, many research groups focus on SiO materials as anode of lithium ion battery, e.g. 1) Jae-Hun Kim et al reported a silicon monoxide/carbon composite prepared by ball-milling of SiO, following with a pyrolysis process. This composite shows a reversible capacity of 710 mAh g−1 over 100 cycles (see Kim, J.-H., et al., Enhanced cycle performance of SiO—C composite anode for lithium-ion batteries. Journal of Power Sources, 2007. 170(2): p. 456-459); 2) Wei-Ren Liu et al reported that carbon-coating of sub-pm SiO particles by a fluidized-bed chemical-vapor-deposition process exhibited a capacity of 620mAh g−1 after 50 cycles (see Liu, W.-R., et al., Nano-porous SiO/Si/Carbon composite anode for lithium-ion batteries. Journal of Applied Electrochemistry, 2009. 39(9): p. 1643-1649); and 3) Jung-In Lee et al reported a silicon-based muticomponent composite composed of porous silicon monoxide, silicon and silica which form from disproportionation reaction of porous silicon monoxide. This composite shows reversible capacity of 1500 mAh g−1 with cycle life of 100 times.
It is well known that commercially available amorphous, solid SiO is technically prepared from Si and SiO2 at high temperatures by condensation of gaseous SiO. The atomic structure of solid SiO is still controversial, despite a number of physical and chemical investigations has been conducted. In addition, the above prior arts disclose that the SiO2 is unavoidable because of disproportionation reaction during the process for producing the SiO-based material. The formation of SiO2 in solid SiO material badly damages the capacity of the material, because SiO2 is inactive toward lithium ion, and therefore contributes no capacity for anode capacity. Moreover, SiO2 decrease silicon content, which is the main contributor of anode capacity.
Accordingly, a need exists for an easy and cost-efficient process for producing a SiOx/Si/C material that exhibits larger capacity, a reduced content of Li2O and Li4SiO4 formed during lithiation process by adjusting the atom ratio of Si:O in the SiOx/Si/C composite material.